The field of the invention is that of the transmission of multicarrier digital signals, that is to say signals implementing a plurality of multicarriers transmitted simultaneously and modulated each by distinct data elements. More specifically, the invention relates to the time synchronization of receivers of such signals.
Multicarrier signals are generally called frequency division multiplex (FDM) signals. A particular example of these signals to which the invention can be applied in particular is that of OFDM (orthogonal frequency division multiplex) signals.
An OFDM signal is used for example in the digital broadcasting system described especially in the French patent FR 86 06922 filed on Jul. 2, 1986 and in M. Alard and R. Lassalle, Principes de modulation et de codage canal en radiodiffusion numxc3xa9rique vers les mobiles (xe2x80x9cPrinciples of modulation and channel encoding in digital radio broadcasting towards mobile unitsxe2x80x9d), Revue de l""UER, No. 224, August 1986, pp. 168-190, known as the COFDM (coded orthogonal frequency division multiplex) system.
This COFDM system has been developed in the context of the European DAB (digital audio broadcasting) project. It is also being put forward for standardization for the terrestrial broadcasting of digital television (the DVB-T standard in particular). More generally, the COFDM system can be used to transmit any type of digital signals (or analog signals that are sampled but not necessarily quantified).
This system is based on the joint use of a channel encoding device and an orthogonal frequency division multiplex method of modulation. This system is particularly suited to the broadcasting of digital signals at a high bit rate (some megabits per second) in channels that are assigned multiple paths whose characteristics vary in time (for example in the case of mobile reception in an urban environment).
The modulation method proper makes it possible to overcome problems related to the frequency selectivity of the channel. It consists in redistributing the information to be transmitted over a large number of carriers juxtaposed and modulated at a low bit rate. A system for the interlacing of information to be transmitted is associated with the encoding method in such a way that the maximum statistical independence of the samples is ensured at the input of the decoder.
The time synchronization of a COFDM receiver consists of the determining, in the frame of the OFDM signal received, of the location of the useful part of each symbol (constituted by a guard interval and a useful part) for the application thereto of the FFT window enabling the selection of the useful part of each symbol. This information of time synchronization is also used for the feedback control of the clock of the receiver in order to implement the rate recovery device.
This time synchronization function of the receiver can be generally subdivided into a rough time synchronization (during the acquisition period) and a fine time synchronization.
According to a known technique, implemented especially in the DAB digital broadcasting program, the time synchronization can be based on special symbols designed for this purpose, generally placed at the beginning of a frame.
In this case, each frame advantageously starts with at least two special symbols, S1 and S2, used for the synchronization. It then comprises a certain number of useful symbols, each comprising a plurality of modulated orthogonal carriers.
The symbol S1 is a zero symbol enabling firstly the performance of a rough synchronization. The symbol S2 is a second synchronization symbol formed by a non-modulated multiplex of all the carrier frequencies with a substantially constant envelope. This enables a more precise recomputation of the synchronization by analysis of the pulse response of the channel. The role and the mode of preparing these symbols S1 and S2 are described in the patent FR 88 15216 filed on Nov. 18, 1988 on behalf of the present Applicants.
The symbol S2 is also known as the CAZAC symbol and the TFPC symbol in other embodiments.
The idea is now being envisaged of producing COFDM signals that do not have such special symbols dedicated to time synchronization. This is especially the case with digital television signals under standardization.
Other synchronization methods should therefore be developed. Thus, there is a known technique, called the guard interval correlation technique which enables the performance of a rough synchronization.
The guard interval of an OFDM symbol consists of the repetition of the samples of the end of said OFDM symbol. The method consists of the computation of the correlation between the samples constituting the guard interval and the samples of the end of the symbol in order to extract a correlation xe2x80x9cpeakxe2x80x9d therefrom.
After temporal filtering, this correlation xe2x80x9cpeakxe2x80x9d can then be used as a synchronization pulse to determine the length of the OFDM symbol and of the guard interval xcex94, and hence the beginning of the FFT window. This operation is performed before the demodulation FFT on the COFDM signal received in the temporal domain.
If x(t) designates the COFDM signal received in the temporal domain, the measurement of the correlation at the instant t=Tn can be given by the following expression:             Γ      x        ⁡          (              T        n            )        =            ∑              t        =                  T          n                                      T          n                +                  T          i                      ⁢          "LeftBracketingBar"                        x          ⁡                      (            t            )                          ·                              x            *                    ⁡                      (                          t              -                              t                s                                      )                              "RightBracketingBar"      
where * signifies the xe2x80x9cconjugatexe2x80x9d of a complete number and | | signifies xe2x80x9cthe modulusxe2x80x9d of a complex number.
The measurement of the correlation is done on blocks with a length Ti equal to or smaller than the length of the guard interval xcex94. Should the receiver not have a priori knowledge of the length of the guard interval (there are provided, in certain systems, variable sizes depending on the application), the measurement of the correlation may be done at the outset on blocks of a length equal to the minimum length of the guard interval.
Under ideal conditions where there is no noise, no multiple paths and no co-channel interference, the correlation xe2x80x9cpeakxe2x80x9d (or xe2x80x9cpulsexe2x80x9d) obtained may be exploited to generate the xe2x80x9croughxe2x80x9d time synchronization.
For example, FIG. 1 shows the measurement of the correlation obtained after temporal filtering in the case of a noise-affected ideal transmission (11) and a noiseless ideal transmission (12) with a pulse response h(t) 13 of the channel with only one path 14.
This information can also be used for the fine time synchronization: by measuring the distance 15 between two successive correlation xe2x80x9cpeaksxe2x80x9d, it is possible to deduce the length Ts=Ts+xcex94 of an OFDM symbol and therefore the length of the guard interval xcex94.
By contrast, in the presence of major echoes or a high level of interference, the correlation peak obtained is highly deformed and is more or less spread as a function of the spread of the echoes.
FIG. 2 shows the measurement of the correlation obtained after temporal filtering in the case of a noise-affected transmission (21) and a noiseless transmission (22) which however is characterized by a pulse response h(t) 23 of the channel having two paths 241 and 242 spaced out by the length of the guard interval xcex94 and received with identical power.
The correlation peak 25 may then be exploited to determine the length of an OFDM symbol and generate the rough time synchronization but its precision is insufficient for the deduction therefrom of a fine temporal synchronization.
Indeed, when the reception conditions evolve in time, which is the case in portable and mobile reception, the form of the measurement of the correlation of the guard interval is highly fluctuating. A fine time synchronization generated solely from this information, even if this information is temporally filtered, will then be affected by a substantial amount of jitter.
Furthermore, the measurement of the correlation of the guard interval will be greatly polluted in the presence of intersymbol interference due to the presence of echoes.
An aim of the invention in particular is to overcome these drawbacks of the prior art.
More specifically, an object of the invention is to provide a method and device of time synchronization for multicarrier signal receivers that do not require the transmission of synchronization symbols specific to this function.
In particular, an aim of the invention is to provide a method of this kind and a device of this kind compatible with the signal structure presently proposed for terrestrial broadcasting of digital television.
Another aim of the invention is to provide a method and device of this kind that makes it possible to to obtain a fine synchronization of high quality, even in the presence of a received signal that is highly disturbed. In particular, it is an aim of the invention to enable accurate operation in the presence of echoes that are long and possibly lengthier than the duration of the guard interval.
Yet another aim of the invention is to provide a method and device of this kind that can easily be implemented in any type of receiver with limited technical complexity and cost price.
These goals as well as others that shall appear hereinafter are achieved according to the invention by a method for the time synchronization of a receiver of a multicarrier signal consisting of a sequence of symbols each formed by a plurality of carrier frequencies each modulated by a modulation coefficient,
certain of said carrier frequencies, with a position in the time-frequency space that is known to said receiver, being reference carrier frequencies bearing a reference coefficient with a value known to said receiver,
said method comprising a fine synchronization phase comprising the following steps:
the estimation of the pulse response ĥn of the transmission channel of said signal on the basis of reference coefficients belonging to at least two received symbols;
the determination of the beginning of the useful part of each of said symbols and/or the feedback control of a clock of the receiver by the analysis of said estimation of the pulse response ĥn.
In other words, the invention proposes the creation of a fictitious time synchronization symbol, in bringing together several reference elements belonging to several symbols. The fictitious symbol thus obtained is used to determine an estimation of the pulse response and deduce a fine synchronization therefrom.
This technique is quite novel and inventive for those skilled in the art who have always felt that it is necessary for at least one particular synchronization symbol to be transmitted to determine an estimation of the pulse response.
Furthermore, it requires adaptations that are not obvious, presented here below when, as is generally the case, the pulse response obtained according to the invention is sub-sampled.
Advantageously, said step for the estimation of the pulse response ĥn comprises the following steps:
the extraction of the reference coefficients belonging to M symbols comprising successive reference carriers, said reference carriers being separated by L useful carriers in one and the same symbol and being offset by R carriers between two symbols comprising successive reference carriers, with M and L greater than or equal to 2 and R=(L+1)/M;
the grouping together of said extracted reference coefficients so as to form a fictitious synchronization symbol, sub-sampled by a factor R;
the reverse Fourier transform of said fictitious synchronization symbol so as to obtain an estimation of the pulse response ĥn extending over a duration ts/R, ts being the useful period of a symbol.
This distribution of the reference carriers (in a quincunxial arrangement) is of course only one example, and many other forms of distribution can be envisaged. If there is no other possibility, the reference carriers of only one symbol may be used but in this case the pulse response is highly sub-sampled.
As mentioned here above, in certain situations, the sub-sampling introduces uncertainties with regard to the synchronization correction to be made (in advance or delayed). To overcome this problem, said analysis of the estimation of the pulse response ĥn advantageously comprises the following steps:
the search for a pulse representing the first path of the signal in the transmission channel;
the analysis of said pulse representing the first path so as to determine if it is an echo or a pre-echo,
and said step for the determination of the start of the useful part of each of said symbols checks the position of a temporal selection window on the received signal, so as to position said pulse representing the first path substantially at the instant t=0 in said estimation of the pulse response ĥn, said window having to be delayed if it is an echo and advanced if it is a pre-echo.
Preferably, in this case, the time range [0, tmax] on which said estimation of the pulse response ĥn extends is divided into two fields:
a first field, called a field in advance, extending from the instant 0 to an instant tlim, and
a second field, called a delayed field, extending from the instant tlim to the instant tmax,
said pulse representing the first path being considered to be an echo if it is in said field in advance and a pre-echo if it is in said delayed field.
tlim may be chosen for example to be greater than or equal to the duration xcex94 of the guard interval preceding the useful part of each of said symbols.
Preferably, the method of the invention also comprises a rough synchronization phase implemented in parallel to said fine synchronization phase, and comprising a step for the correlation of the guard interval (known per se and described in the introduction) consisting of a search for a correlation peak between the contents of a guard interval preceding the useful part of each of said symbols and the end of the useful part of said symbol so as to determine the duration of said symbol.
Apart from the rough synchronization, the correlation of the guard interval may be used in a novel fashion to further improve the fine synchronization of the invention, in certain special cases.
Indeed, advantageously, the method then comprises a step for distinguishing between a long echo and a pre-echo, when said pulse representing the first path is in said delayed field, by analysis of the measurement of the correlation of the guard interval, consisting in determining the number of samples of said measurement of the correlation greater than a decision threshold, said pulse representing the first path being considered as a long echo when said number of samples is greater than a predetermined value.
This technique makes it possible to remove the uncertainty of certain borderline cases between a very long echo and a pre-echo.
Advantageously, said decision threshold is substantially proportional to the smallest amplitude of the two amplitudes corresponding to the highest pulse of each of said fields that are xe2x80x98in advancexe2x80x99 or xe2x80x98delayedxe2x80x99.
The invention also relates to the devices implementing the method described here above. A time synchronization device of this kind for a receiver of a multicarrier signal comprises fine synchronization means comprising:
means for the estimation of the pulse response ĥn of the transmission channel of said signal on the basis of reference coefficients belonging to at least two received symbols;
means to determine the beginning of the useful part of each of said symbols by the analysis of said estimation of the pulse response ĥn.